An assessment of natural radioactivity of soils and its external radiological impact in southwestern Nigeria.

Soil samples collected from five states in the southwestern part of Nigeria have been analyzed for activity concentration of gamma-ray emitters. The States were Oyo, Lagos, Ondo, Ogun, and Osun. Activity concentration measurement was carried out using a co-axial type high purity germanium (HPGe) detector. Spectral analyses were performed using spectrometry software. The activity concentration measured ranged from 0.03 +/- 0.2 Bq kg(-1) for 137Cs to 1,358.6 +/- 28.5 Bq kg(-1) for (40)K. (40)K accounted for most of the activity. The calculated absorbed dose rates in air at 1.0 m above the ground ranged from 6 to 303 nGy h(-1) with a mean and standard deviation of 60.5 nGy h(-1) and 63.2 nGy h(-1), respectively. Estimated annual outdoor effective dose equivalent varied from 8 to 370 mSv y(-1). The calculated collective dose equivalent values for the five states ranged from 87,000 person-Sv to 600,000 person-Sv. The estimated annual outdoor effective values were considerably higher than the world soil average value of 70 microSv y(-1) reported by the International Commission on Radiological Protection.